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242                                                 ZINC

and iron sulphides, these being estimated as under a, b and c (below) ; the
copper and silver sulphides, which remain undissolved, may be dissolved
in nitric acid and estimated by the ordinary methods.

(a)  DETERMINATION OF THE LEAD.   The hydrochloric acid solution is
evaporated in presence of sulphuric acid and heated until white fumes
appear, the cold residue being taken up in water and alcohol added ; the
subsequent  procedure  is as  in the   gravimetric   determination   of  lead
in brass (see p. 226).

(b)  DETERMINATION OF THE CADMIUM.    The liquid  freed from lead
sulphate is evaporated until the alcohol is expelled, neutralised exactly
with ammonia, treated for every 100 c.c. of liquid with 10 c.c. of 25%
hydrochloric acid (D 1-125) and then with hydrogen sulphide.    The pre-
cipitate is filtered off, washed with saturated hydrogen sulphide solution
and dissolved in nitric acid (D 1-2), the solution being evaporated in a
tared platinum crucible in presence of a slight excess of sulphuric acid,
the excess of the latter being eliminated and the residue heated gently and
weighed :   CdS04 X 0-5392 = Cd.

(c)  DETERMINATION OF THE IRON.   The excess of hydrogen sulphide
is eliminated from the filtrate from the cadmium sulphide, the iron being
oxidised by a few drops of hydrogen peroxide or bromine water, and ammonia
added until the reaction is alkaline.    After a short rest on a water-bath,
the precipitated ferric hydroxide is filtered and washed.    To free the pre-
cipitate from all traces of zinc, it is redissolved in dilute hydrochloric acid,
again precipitated with ammonia, filtered off, washed with slightly ammo-
niacal water, dried, ignited and weighed :   Fe303 x 0-6994 = Fe.

2. Determination of the Sulphur, Antimony and Arsenic (accord-
ing to Gunther *} .  100 grams of the sample are placed in a large flask
furnished with a delivery tube and with a tapped funnel, the stem of
which, bent up at the end, reaches almost to the bottom of the vessel. The
air is expelled by means of pure hydrogen washed by passing it through
silver nitrate solution and the delivery tube connected with two washing
bottles, the first containing a solution of cadmium cyanide in potassium
cyanide and the second silver nitrate solution, pure dilute sulphuric acid
being then gradually introduced by means of the tapped funnel until the
metal dissolves completely. When evolution of gas ceases, a moderate
current of hydrogen is passed through the apparatus to displace the gaseous
products of the reaction. The first washing bottle retains the sulphur as
cadmium sulphide, which is filtered off and converted into sulphate (see i)
and weighed.

In the second bottle, if hydrogen arsenide and antimonide are present,
metallic silver and silver antimonide 2 separate. The precipitate is filtered
off, washed and dissolved in nitric and tartaric acids, the silver being then
precipitated as chloride, which is filtered off, washed, dried and weighed.
In the filtrate, the greater part of the acidity is neutralised with ammonia

1  Zeitschr. analyt. Chem., 1881, XX, p. 503.

2  The reaction occurs according to the equations :

SbH3 + 3AgNO3   + aq.      = Ag3Sb
2AsH3 + i2AgN03 + 3H2O = i2Ag   + I2HNO3

As2O3.cal Chemistry, Vol. II.to flow immediately the elec-